There is proposed a technology for making a failure determination of an intake throttle valve (valve device that generates negative pressure of intake air at a merging part of the intake passage and the low-pressure EGR passage) by means of a low-pressure EGR opening sensor that detects an opening degree of a low-pressure EGR regulating valve (valve device that regulates an opening degree of a low-pressure EGR passage leading to an intake passage) (see JP-A-2011-032929 corresponding to US2011/0023846A1).
According to this technology described in JP-A-2011-032929, mechanical stopper which restricts a limit opening degree of a linking device (specifically, engagement part between an end portion of a cam groove and a roller) is provided on a fully-closed side of the low-pressure EGR regulating valve in the linking device. After an engine stops, the low-pressure EGR regulating valve is rotated on the fully-closed side, and the movement of the linking device is thereby stopped by the mechanical stopper. In other words, the low-pressure EGR regulating valve is rotated on the fully-closed side up to a range in which the movement of the linking device is restricted by the mechanical stopper. If the opening degree detected by the low-pressure EGR opening sensor is the limit opening degree of the mechanical stopper, it is determined that the intake throttle valve is normal. If it is an opening degree that is different from the limit opening degree, it is determined that a failure has occurred in the intake throttle valve.
According to this technology in JP-A-2011-032929, the fully-closed position of the EGR valve and the limit opening degree of the mechanical stopper may be set at the same opening degree. The low-pressure EGR regulating valve is very often returned to the fully-closed position when the engine is in operation as well as at the time of the engine stop. For this reason, each time the low-pressure EGR regulating valve is returned to the fully-closed position, stress (pressing load) is applied to the mechanical stopper. As a result, the stress is applied to the mechanical stopper many times for a long period of time, and there is concern that strength of the mechanical stopper is reduced.